This invention relates to a pseudorandom (PN) code tracking loop, and more particularly to a technique for providing an indication of lock in a PN code tracking loop.
Modulation of a carrier with PN codes is often used to spread the carrier (and command/data sideband) energy over a wide bandwidth for anti-jamming, security or radiated energy density reduction purposes. In order to despread and demodulate these signals a receiver must correlate and phase track an identical local code with the code used to spread the signal.
In the past, a receiver which must receive a signal spread by PN-code modulation has used a PN code tracking loop at IF that dithers a local PN code (i.e., that continually advances and retards the PN code in phase a half code bit period hereinafter referred to as a "half chip") at some low frequency, typically at 200 Hertz. The PN-spread IF signal is correlated (multiplied) with the dithered PN and bandpass filtered to produce a partially despread IF signal. This despread IF signal is then amplified and square-law detected to obtain a phase error (DC) signal proportional to the degree of correlation. The phase error signal is applied to a voltage controlled crystal oscillator (VCXO) which in turn controls the phase of the PN code generator, but without giving any indication of PN code tracking loop lock. An object of this invention is to provide an indication of lock in a dithered PN code tracking loop.